Method of processing a workpiece using an externally excited torroidal plasma source

ABSTRACT

A method of processing a workpiece in a plasma reactor includes establishing a torroidal path for a plasma current to flow that passes near and transverse to the surface of said workpiece, maintaining a plasma current in the torroidal path by applying RF power to a portion of the torroidal path away from the surface of the workpiece, and increasing the ion density of the plasma current in the vicinity of the workpiece by constricting the area of a portion of the torroidal path overlying the workpiece.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention concerns plasma reactors used in processing workpieces inthe manufacturing of items such as microelectronic circuits, flat paneldisplays and the like, and in particular plasma sources therefor.

2. Background Art

The trend in microelectronic circuits toward ever increasing densitiesand smaller feature sizes continues to make plasma processing of suchdevices more difficult. For example, the diameter of contact holes hasbeen reduced while the hole depth has increased. During plasma-enhancedetching of a dielectric film on a silicon wafer, for example, the etchselectivity of the dielectric material (e.g. silicon dioxide) tophotoresist must be sufficient to allow the etch process to etch acontact hole whose diameter is ten to fifteen times its depth, withoutappreciably disturbing the photoresist mask defining the hole. This taskis made even more difficult because the recent trend toward shorterwavelength light for finer photolithography requires a thinnerphotoresist layer, so that the dielectric-to-photoresist etchselectivity must be greater than ever. This requirement is more readilymet using processes having relatively low etch rates, such as dielectricetch processes employing a capacitively coupled plasma. The plasmadensity of a capacitively coupled plasma is relatively less than that ofan inductively coupled plasma, and the capacitively coupled plasma etchprocess exhibits good dielectric-to-photoresist etch selectivity. Theproblem with the capacitively coupled process is that it is slow andtherefore relatively less productive. Another problem that arises insuch etch processes is non-uniform plasma distribution.

In order to increase productivity or etch rate, higher density plasmashave been employed. Typically, the high density plasma is an inductivelycoupled plasma. However, the process precursor gases tend to dissociatemore rapidly in such a high density plasma, creating a higher plasmacontent of free fluorine, a species which reduces the etch selectivityto photoresist. To reduce this tendency, fluoro-carbon process gasessuch as CF₂ are employed which dissociate in a plasma intofluorine-containing etchant species and one or more polymer specieswhich tend to accumulate on non-oxide containing surfaces such asphotoresist. This tends to increase etch selectivity. The oxygen in theoxygen-containing dielectric material promotes the pyrolization of thepolymer over the dielectric, so that the polymer is removed, allowingthe dielectric material to be etched while the non-oxygen containingmaterial (e.g., the photoresist) continues to be covered by the polymerand therefore protected from the etchant. The problem is that theincrease in contact opening depth and decrease in photoresist thicknessto accommodate more advanced device designs has rendered the highdensity plasma process more likely to harm the photoresist layer duringdielectric etching. As the plasma density is increased to improve etchrate, a more polymer-rich plasma must be used to protect the non-oxygencontaining material such as photoresist, so that the rate of polymerremoval from the oxygen-containing dielectric surfaces slowsappreciably, particularly in small confined regions such as the bottomof a narrow contact opening. The result is that, while the photoresistmay be adequately protected, the possibility is increased for the etchprocess to be blocked by polymer accumulation once a contact openingreaches a certain depth. Typically, the etch stop depth is less than therequired depth of the contact opening so that the device fails. Thecontact opening may provide connection between an upper polysiliconconductor layer and a lower polysilicon conductor layer through anintermediate insulating silicon dioxide layer. Device failure occurs,for example, where the etch stop depth is less than the distance betweenthe upper and lower polysilicon layers. Alternatively, the possibilityarises of the process window for achieving a high density plasma withoutetch stop becoming too narrow for practical or reliable application tothe more advanced device designs such as those having contact openingswith aspect ratios of 10:1 or 15:1.

What would be desirable at present is a reactor that has the etch rateof an inductively coupled plasma reactor (having a high density plasma)with the selectivity of a capacitively coupled reactor. It has beendifficult to realize the advantages of both types of reactors in asingle machine led reactor.

One problem with high density inductively coupled plasma reactors,particularly of the type having an overhead coil antenna facing thewafer or workpiece, is that as the power applied to the coil antenna isincreased to enhance the etch rate, the wafer-to-ceiling gap must besufficiently large so that the power is absorbed in the plasma regionwell above the wafer. This avoids a risk of device damage on-the waferdue to strong RF fields. Moreover, for high levels of RF power appliedto the overhead coil antenna, the wafer-to-ceiling gap must berelatively large, and therefore the benefits of a small gap cannot berealized.

If the ceiling is a semiconductive window for the RF field of aninductively coupled reactor, or a conductive electrode of a capacitivelycoupled reactor, then one benefit of a small wafer-to-ceiling gap is anenhanced electric potential or ground reference that the ceiling couldprovide across the plane of the wafer at a relatively small gap distance(e.g., on the order of 1 or 2 inches).

Therefore, it would be desireable to have a reactor not only having theselectivity of a capacitively coupled reactor with the ion density andetch rate of an inductively coupled reactor, but further having none ofthe conventional limitations on the wafer-to-ceiling gap length otherthan a fundamental limit, such as the plasma sheath thickness, forexample. It would further be desireable to have a reactor having theselectivity of a capacitively coupled reactor and the etch rate of aninductively coupled reactor in which the ion density and etch rate canbe enhanced without necessarily increasing the applied RF plasma sourcepower.

SUMMARY OF THE DISCLOSURE

A method of processing a workpiece in a plasma reactor includesestablishing a torroidal path for a plasma current to flow that passesnear and transverse to the surface of said workpiece, maintaining aplasma current in the torroidal path by applying RF power to a portionof the torroidal path away from the surface of the workpiece, andincreasing the ion density of the plasma current in the vicinity of theworkpiece by constricting the area of a portion of the torroidal pathoverlying the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a first embodiment that maintains an overheadtorroidal plasma current path.

FIG. 2 is a side view of an embodiment corresponding to the embodimentof FIG. 1.

FIG. 3 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in wafer-to-ceiling gapdistance.

FIG. 4 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF bias power applied tothe workpiece.

FIG. 5 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in RF source power appliedto the coil antenna.

FIG. 6 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in reactor chamber pressure.

FIG. 7 is a graph illustrating the behavior of free fluorineconcentration in the plasma with variations in partial pressure of aninert diluent gas such as Argon.

FIG. 8 is a graph illustrating the degree of dissociation of process gasas a function of source power for an inductively coupled reactor and fora reactor of the present invention.

FIG. 9 illustrates a variation of the embodiment of FIG. 1.

FIGS. 10 and 11 illustrate a variation of the embodiment of FIG. 1 inwhich a closed magnetic core is employed.

FIG. 12 illustrates another embodiment of the invention in which atorroidal plasma current path passes beneath the reactor chamber.

FIG. 13 illustrates a variation of the embodiment of FIG. 10 in whichplasma source power is applied to a coil wound around a distal portionthe closed magnetic core.

FIG. 14 illustrates an embodiment that establishes two paralleltorroidal plasma currents.

FIG. 15 illustrates an embodiment that establishes a plurality ofindividually controlled parallel torroidal plasma currents.

FIG. 16 illustrates a variation of the embodiment of FIG. 15 in whichthe parallel torroidal plasma currents enter and exit the plasma chamberthrough the vertical side wall rather than the ceiling.

FIG. 17A illustrates an embodiment that maintains a pair of mutuallyorthogonal torroidal plasma currents across the surface of theworkpiece.

FIG. 17B illustrates the use of plural radial vanes in the embodiment ofFIG. 17A.

FIGS. 18 and 19 illustrate an embodiment of the invention in which thetorroidal plasma current is a broad belt that extends across a wide pathsuitable for processing large wafers.

FIG. 20 illustrates a variation of the embodiment of FIG. 18 in which anexternal section of the torroidal plasma current path is constricted.

FIG. 21 illustrates a variation of the embodiment of FIG. 18 employingcylindrical magnetic cores whose axial positions may be adjusted toadjust ion density distribution across the wafer surface.

FIG. 22 illustrates a variation of FIG. 21 in which a pair of windingsare wound around a pair of groups of cylindrical magnetic cores.

FIG. 23 illustrates a variation of FIG. 22 in which a single commonwinding is wound around both groups of cores.

FIG. 24 and 25 illustrate an embodiment that maintains a pair ofmutually orthogonal torroidal plasma currents which are wide beltssuitable for processing large wafers.

FIG. 26 illustrates a variation of the embodiment of FIG. 25 in whichmagnetic cores are employed to enhance inductive coupling.

FIG. 27 illustrates a modification of the embodiment of FIG. 24 in whichthe orthogonal plasma belts enter and exit the reactor chamber throughthe vertical side wall rather than through the horizontal ceiling.

FIG. 28A illustrates an implementation of the embodiment of FIG. 24which produces a rotating torroidal plasma current.

FIG. 28B illustrates a version of the embodiment of FIG. 28A thatincludes magnetic cores.

FIG. 29 illustrates a preferred embodiment of the invention in which acontinuous circular plenum is provided to enclose the torroidal plasmacurrent.

FIG. 30 is a top sectional view corresponding to FIG. 29.

FIGS. 31A and 31B are front and side sectional views corresponding toFIG. 30.

FIG. 32 illustrates a variation of the embodiment 29 employing threeindependently driven RF coils underneath the continuous plenum facing at120 degree intervals.

FIG. 33 illustrates a variation of the embodiment of FIG. 32 in whichthe three RF coils are driven at 120 degree phase to provide anazimuthally rotating plasma.

FIG. 34 illustrates a variation of the embodiment of FIG. 33 in which RFdrive coils are wound around vertical external ends of respectivemagnetic cores whose opposite ends extend horizontally under the plenumat symmetrically distributed angles.

FIG. 35 is an version of the embodiment of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the embodiment of FIG. 20.

FIG. 36 is a version of the embodiment of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is an embodiment corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 38 is an embodiment corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 39 is an embodiment corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

FIG. 40 is an embodiment corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

FIG. 41 is an embodiment corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

FIG. 42 is an embodiment corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

FIG. 43 is an embodiment corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Overview of the Plasma Reactor Chamber

Referring to FIG. 1, a plasma reactor chamber 100 enclosed by acylindrical side wall 105 and a ceiling 110 houses a wafer pedestal 115for supporting a semiconductor wafer or workpiece 120. A process gassupply 125 furnishes process gas in to the chamber 100 through gas inletnozzles 130 a-130 d extending through the side wall 105. A vacuum pump135 controls the pressure within the chamber 100, typically holding thepressure below 0.5 milliTorr (mT). A half-torroidal hollow tubeenclosure or conduit 150 extends above the ceiling 110 in a half circle.The conduit 150, although extending externally outwardly from ceiling110, is nevertheless part of the reactor and forms a wall of thechamber. Internally it shares the same evacuated atmosphere as existselsewhere in the reactor. In fact, the vacuum pump 135, instead of beingcoupled to the bottom of the main part of the chamber as illustrated inFIG. 1, may instead be coupled to the conduit 150, although this is notpresently preferred. The conduit 150 has one open end 150 a sealedaround a first opening 155 in the reactor ceiling 110 and its other end150 b sealed around a second opening 160 in the reactor ceiling 110. Thetwo openings or ports 150, 160 are located on generally opposite sidesof the wafer support pedestal 115. The hollow conduit 150 is reentrantin that it provides a flow path which exits the main portion of thechamber at one opening and re-enters at the other opening. In thisspecification, the conduit 150 may be described as being half-torroidal,in that the conduit is hollow and provides a portion of a closed path inwhich plasma may flow, the entire path being completed by flowing acrossthe entire process region overlying the wafer support pedestal 115.Notwithstanding the use of the term “torroidal”, the trajectory of thepath as well as the cross-sectional shape of the path or conduit 150 maybe circular or non-circular, and may be square, rectangular or any othershape either a regular shape or irregular.

The external conduit 150 may be formed of a relatively thin conductorsuch as sheet metal, but sufficiently strong to withstand the vacuumwithin the chamber. In order to suppress eddy currents in the sheetmetal of the hollow conduit 150 (and thereby facilitate coupling of anRF inductive field into the interior of the conduit 150), an insulatinggap 152 extends across and through the hollow conduit 150 so as toseparate it into two tubular sections. The gap 152 is filled by a ring154 of insulating material such as a ceramic in lieu of the sheet metalskin, so that the gap is vacuum tight. A second insulating gap 153 maybe provided, so that one section of the conduit 150 is electricallyfloating. A bias RF generator 162 applies RF bias power to the waferpedestal 115 and wafer 120 through an impedance match element 164.

Alternatively, the hollow conduit 150 may be formed of a non-conductivematerial instead of the conductive sheet metal. The non-conductivematerial may be a ceramic, for example. In such an alternativeembodiment, neither gap 152 or 153 is required.

An antenna 170 such as a winding or coil 165 disposed on one side of thehollow conduit 150 and wound around an axis parallel to the axis ofsymmetry of the half-torroidal tube is connected through an impedancematch element 175 to an RF power source 180. The antenna 170 may furtherinclude a second winding 185 disposed on the opposite side of the hollowconduit 150 and wound in the same direction as the first winding 165 sothat the magnetic fields from both windings add constructively.

Process gases from the chamber 100 fill the hollow conduit 150. Inaddition, a separate process gas supply 190 may supply process gasesdirectly in to the hollow conduit 150 through a gas inlet 195. The RFfield in the external hollow conduit 150 ionizes the gases in the tubeto produce a plasma. The RF field induced by the circular coil antenna170 is such that the plasma formed in the tube 150 reaches through theregion between the wafer 120 and the ceiling 110 to complete a torroidalpath that includes the half-torroidal hollow conduit 150. As employedherein, the term “torroidal” refers to the closed and solid nature ofthe path, but does not refer or limit its cross-sectional shape ortrajectory, either of which may be circular or non-circular or square orotherwise. Plasma circulates through the complete torroidal path orregion which may be thought of as a closed plasma circuit. The torroidalregion extends across the diameter of the wafer 120 and, in certainembodiments, has a sufficient width in the plane of the wafer so that itoverlies the entire wafer surface.

The RF inductive field from the coil antenna 170 includes a magneticfield which itself is closed (as are all magnetic fields), and thereforeinduces a plasma current along the closed torroidal path described here.It is believed that power from the RF inductive field is absorbed atgenerally every location along the closed path, so that plasma ions aregenerated all along the path. The RF power absorption and rate of plasmaion generation may vary among different locations along the closed pathdepending upon a number of factors. However, the current is generallyuniform along the closed path length, although the current density mayvary. This current alternates at the frequency of the RF signal appliedto the antenna 170. However, since the current induced by the RFmagnetic field is closed, the current must be conserved around thecircuit of the closed path, so that the amount of current flowing in anyportion of the closed path is generally the same as in any other portionof the path. As will be described below, this fact is exploited in theinvention to great advantage.

The closed torroidal path through which the plasma current flows isbounded by plasma sheaths formed at the various conductive surfacesbounding the path. These conductive surfaces include the sheet metal ofthe hollow conduit 150, the wafer (and/or the wafer support pedestal)and the ceiling overlying the wafer. The plasma sheaths formed on theseconductive surfaces are charge-depleted regions produced as the resultof the charge imbalance due to the greater mobility of the low-massnegative electrons and the lesser mobility of the heavy-mass positiveions. Such a plasma sheath has an electric field perpendicular to thelocal surface underlying the sheath. Thus, the RF plasma current thatpasses through the process region overlying the wafer is constricted byand passes between the two electric fields perpendicular to the surfaceof the ceiling facing the wafer and the surface of the wafer facing thegas distribution plate. The thickness of the sheath (with RF biasapplied to the workpiece or other electrode) is greater where theelectric field is concentrated over a small area, such as the wafer, andis less in other locations such as the sheath covering the ceiling andthe large adjoining chamber wall surfaces. Thus, the plasma sheathoverlying the wafer is much thicker. The electric fields of the waferand ceiling/gas distribution plate sheaths are generally parallel toeach other and perpendicular to the direction of the RF plasma currentflow in the process region.

When RF power is first applied to the coil antenna 170, a dischargeoccurs across the gap 152 to ignite a capacitively coupled plasma fromgases within the hollow conduit 150. Thereafter, as the plasma currentthrough the hollow conduit 150 increases, the inductive coupling of theRF field becomes more dominant so that the plasma becomes an inductivelycoupled plasma. Alternatively, plasma may be initiated by other means,such as by RF bias applied to the workpiece support or other electrode.

In order to avoid edge effects at the wafer periphery, the ports 150,160 are separated by a distance that exceeds the diameter of the wafer.For example, for a 12 inch diameter wafer, the ports 150, 160 are about16 to 22 inches apart. For an 8 inch diameter wafer, the ports 150, 160are about 10 to 16 inches apart.

Advantages of the Invention

A significant advantage is that power from the RF inductive field isabsorbed throughout the relatively long closed torroidal path (i.e.,long relative to the gap length between the wafer and the reactorceiling), so that RF power absorption is distributed over a large area.As a result, the RF power in the vicinity of the wafer-to-ceiling gap(i.e., the process region 121 best shown in FIG. 2, not to be confusedwith the insulating gap 152) is relatively low, thus reducing theliklihood of device damage from RF fields. In constrast, in priorinductively coupled reactors, all of the RF power is absorbed within thenarrow wafer-to-ceiling gap, so that it is greatly concentrated in thatregion. Moreover, this fact often limits the ability to narrow thewafer-to-ceiling gap (in the quest of other advantages) or,alternatively, requires greater concentration of RF power in the regionof the wafer. Thus, the invention overcomes a limitation of longstanding in the art. This aspect enhances process performance byreducing residency time of the reactive gases through a dramaticreduction in volume of the process region or process zone overlying thewafer, as discussed previously herein.

A related and even more important advantage is that the plasma densityat the wafer surface can be dramatically increased without increasingthe RF power applied to the coil antenna 170 (leading to greaterefficiency). This is accomplished by reducing the cross-sectional areaof the torroidal path in the vicinity of the pedestal surface and wafer120 relative to the remainder of the torroidal path. By so constrictingthe torroidal path of the plasma current near the wafer only, thedensity of the plasma near the wafer surface is increasedproportionately. This is because the torroidal path plasma currentthrough the hollow conduit 150 must be at least nearly the same as theplasma current through the pedestal-to-ceiling (wafer-to-ceiling) gap.

A significant difference over the prior art is that not only is the RFfield remote from the workpiece, and not only can ion density beincreased at the wafer surface without increasing the applied RF field,but the plasma ion density and/or the applied RF field may be increasedwithout increasing the minimum wafer-to-ceiling gap length. Formerly,such an increase in plasma density necessitated an increase in thewafer-to-ceiling gap to avoid strong fields at the wafer surface. Incontrast, in the present invention the enhanced plasma density isrealized without requiring any increase in the wafer-to-ceiling gap toavoid a concomitant increase in RF magnetic fields at the wafer surface.This is because the RF field is applied remotely from the wafer andmoreover need not be increased to realize an increase in plasma densityat the wafer surface. As a result, the wafer-to-ceiling gap can bereduced down to a fundamental limit to achieve numerous advantages. Forexample, if the ceiling surface above the wafer is conductive, thenreducing the wafer-to-ceiling gap improves the electrical or groundreference provided by the conductive ceiling surface. A fundamentallimit on the minimum wafer-to-ceiling gap length is the sum of thethicknesses of the plasma sheaths on the wafer surface and on theceiling surface.

A further advantage of the invention is that because the RF inductivefield is applied along the entire torroidal path of the RF plasmacurrent (so that its absorption is distributed as discussed above), thechamber ceiling 110, unlike with most other inductively poweredreactors, need not function as a window to an inductive field andtherefore may be formed of any desired material, such as a highlyconductive and thick metal, and therefore may comprise a conductive gasdistribution plate as will be described below, for example. As a result,the ceiling 110 readily provides a reliable electric potential or groundreference across the entire plane of the pedestal or wafer 120.

Increasing the Plasma Ion Density

One way of realizing higher plasma density near the wafer surface byreducing plasma path cross-sectional area over the wafer is to reducethe wafer-to-ceiling gap length. This may be accomplished by simplyreducing the ceiling height or by introducing a conductive gasdistribution plate or showerhead over the wafer, as illustrated in FIG.2. The gas distribution showerhead 210 of FIG. 2 consists of a gasdistribution plenum 220 connected to the gas supply 125 andcommunicating with the process region over the wafer 120 through pluralgas nozzle openings 230. The advantage of the conductive showerhead 210is two-fold: First, by virtue of its close location to the wafer, itconstricts the plasma path over the wafer surface and thereby increasesthe density of the plasma current in that vicinity. Second, it providesa uniform electrical potential reference or ground plane close to andacross the entire wafer surface.

Preferably, in order to avoid arcing across the openings 230, eachopening 230 is relatively small, on the order of a millimeter (preferredhole diameter is approximately 0.5 mm). The spacing between adjacentopenings may be on the order of a several millimeters.

The conductive showerhead 210 constricts the plasma current path ratherthan providing a short circuit through itself because a plasma sheath isformed around the portion of the showerhead surface immersed in theplasma. The sheath has a greater impedance to the plasma current thanthe space between the wafer 120 and the showerhead 210, and thereforeall the plasma current goes around the conductive showerhead 210.

It is not necessary to employ a showerhead (e.g., the showerhead 210) inorder to constrict the torroidal plasma current or path in the vicinityof the process region overlying the wafer. The path constriction andconsequent increase in plasma ion density in the process region may beachieved without the showerhead 210 by similarly reducing thewafer-to-ceiling height. If the showerhead 210 is eliminated in thismanner, then the process gases may be supplied into the chamber interiorby means of conventional gas inlet nozzles (not shown).

One advantage of the showerhead 210 is that different mixtures ofreactive and inert process gas ratios may be introduced throughdifferent orifices 230 at different radii, in order to finely adjust theuniformity of plasma effects on photoresist, for example. Thus, forexample, a greater proportion of inert gas to reactive gas may besupplied to the orifices 230 lying outside a median radius while agreater proportion of reactive gas to inert gas may be supplied to theorifices 230 within that median radius.

As will be described below, another way in which the torroidal plasmacurrent path may be constricted in the process region overlying thewafer (in order to increase plasma ion density over the wafer) is toincrease the plasma sheath thickness on the wafer by increasing the RFbias power applied to the wafer support pedestal. Since as describedpreviously the plasma current across the process region is confinedbetween the plasma sheath at the wafer surface and the plasma sheath atthe ceiling (or showerhead) surface, increasing the plasma sheaththickness at the wafer surface necessarily decreases the cross-sectionof the portion of the torroidal plasma current within process region,thereby increasing the plasma ion density in the process region. Thus,as will be described more fully later in this specification, as RF biaspower on the wafer support pedestal is increased, plasma ion densitynear the wafer surface is increased accordingly.

High Etch Selectivity at High Etch Rates

The invention solves the problem of poor etch selectivity whichsometimes occurs with a high density plasma. The reactor of FIGS. 1 and2 has a silicon dioxide-to-photoresist etch selectivity as high as thatof a capacitively coupled plasma reactor (about 7:1) while providinghigh etch rates approaching that of a high density inductively coupledplasma reactor. It is believed that the reason for this success is thatthe reactor structure of FIGS. 1 and 2 reduces the degree ofdissociation of the reactive process gas, typically a fluorocarbon gas,so as to reduce the incidence of free fluorine in the plasma region overthe wafer 120. Thus, the proportion of free fluorine in the plasmarelative to other species dissociated from the fluorocarbon gas isdesireably reduced. Such other species include the protectivecarbon-rich polymer precursor species formed in the plasma from thefluorocarbon process gas and deposited on the photoresist as aprotective polymer coating. They further include less reactive etchantspecies such as CF and CF₂ formed in the plasma from the fluorocarbonprocess gas. Free fluorine tends to attack photoresist and theprotective polymer coating formed thereover as vigorously as it attackssilicon dioxide, thus reducing oxide-to-photoresist etch selectivity. Onthe other hand, the less reactive etch species such as CF₂ or CF tend toattack photoresist and the protective polymer coating formed thereovermore slowly and therefore provide superior etch selectivity.

It is believed the reduction in the dissociation of the plasma speciesto free fluorine is accomplished in the invention by reducing theresidency time of the reactive gas in the plasma. This is because themore complex species initially dissociated in the plasma from thefluorocarbon process gas, such as CF₂ and CF are themselves ultimatelydissociated into simpler species including free fluorine, the extent ofthis final step of dissociation depending upon the residency time of thegas in the plasma. The term “residency time” or “residence time” asemployed in this specification corresponds generally to the average timethat a process gas molecule and the species dissociated from the thatmolecule are present in the process region overlying the workpiece orwafer. This time or duration extends from the initial injection of themolecule into the process region until the molecule and/or itsdissociated progeny are pass out of the process region along the closedtorroidal path described above that extends through the processing zone.

As stated above, the invention enhances etch selectivity by reducing theresidency time in the process region of the fluorocarbon process gas.The reduction in residency time is achieved by constricting the plasmavolume between the wafer 120 and the ceiling 110.

The reduction in the wafer-to-ceiling gap or volume has certainbeneficial effects. First, it increases plasma density over the wafer,enhancing etch rate. Second, residency time falls as the volume isdecreased. As referred to above, the small volume is made possible inthe present invention because, unlike conventional inductively coupledreactors, the RF source power is not deposited within the confines ofthe process region overlying the wafer but rather power deposition isdistributed along the entire closed torroidal path of the plasmacurrent. Therefore, the wafer-to-ceiling gap can be less than a skindepth of the RF inductive field, and in fact can be so small as tosignificantly reduce the residency time of the reactive gases introducedinto the process region, a significant advantage.

There are two ways of reducing the plasma path cross-section andtherefore the volume over the wafer 120. One is to reduce thewafer-to-showerhead gap distance. The other is to increase the plasmasheath thickness over the wafer by increasing the bias RF power appliedto the wafer pedestal 115 by the RF bias power generator 162, as brieflymentioned above. Either method results in a reduction in free fluorinecontent of the plasma in the vicinity of the wafer 120 (and consequentincrease in dielectric-to-photoresist etch selectivity) as observedusing optical emission spectroscopy (OES) techniques.

There are three additional methods of the invention for reducing freefluorine content to improve etch selectivity. One method is to introducea non-chemically reactive diluent gas such as argon into the plasma.Preferably, the argon gas is introduced outside and above the processregion by injecting it directly into the hollow conduit 150 from thesecond process gas supply 190, while the chemically reactive processgases (fluorocarbon gases) enter the chamber only through the showerhead210. With this advantageous arrangement, the argon ions, neutrals, andexcited neutrals propagate within the torroidal path plasma current andthrough the process region across the wafer surface to dilute the newlyintroduced reactive (e.g., fluorocarbon) gases and thereby effectivelyreduce their residency time over the wafer. Another method of reducingplasma free fluorine content is to reduce the chamber pressure. Afurther method is to reduce the RF source power applied to the coilantenna 170.

FIG. 3 is a graph illustrating a trend observed in the invention inwhich the free fluorine content of the plasma decreases as thewafer-to-showerhead gap distance is decreased. FIG. 4 is a graphillustrating that the free fluorine content of the plasma is decreasedby decreasing the plasma bias power applied to the wafer pedestal 115.FIG. 5 is a graph illustrating that plasma free fluorine content isreduced by reducing the RF source power applied to the coil antenna 170.FIG. 6 is a graph illustrating that the free fluorine content is reducedby reducing chamber pressure. FIG. 7 is a graph illustrating that plasmafree fluorine content is reduced by increasing the diluent (Argon gas)flow rate into the tubular enclosure 150. The graphs of FIGS. 3-7 aremerely illustrative of plasma behavioral trends inferred from numerousOES observations and do not depict actual data.

Wide Process Window of the Invention

Preferably, the chamber pressure is less than 0.5 T and can be as low as1 mT. The process gas may be C₄F₈ injected into the chamber 100 throughthe gas distribution showerhead at a flow rate of about 15 cc/m with 150cc/m of Argon, with the chamber pressure being maintained at about 20mT. Alternatively, the Argon gas flow rate may be increased to 650 cc/mand the chamber pressure to 60 mT. The antenna 170 may be excited withabout 500 Watts of RF power at 13 MHz. The wafer-to-showerhead gap maybe about 0.3 inches to 2 inches. The bias RF power applied to the waferpedestal may be 13 MHz at 2000 Watts. Other selections of frequency maybe made. The source power applied to the coil antenna 170 may be as lowas 50 KHz or as high as several times 13 MHz or higher. The same is trueof the bias power applied to the wafer pedestal.

The process window for the reactor of FIGS. 1 and 2 is far wider thanthe process window for a conventional inductively coupled reactor. Thisis illustrated in the graph of FIG. 8, showing the specific neutral fluxof free fluorine as a function of RF source power for a conventionalinductive reactor and for the reactor of FIGS. 1 and 2. For theconventional inductively coupled reactor, FIG. 8 shows that the freefluorine specific flux begins to rapidly increase as the source powerexceeds between 50 and 100 Watts. In contrast, the reactor of FIGS. 1and 2 can accept source power levels approaching 1000 Watts before thefree fluorine specific flux begins to increase rapidly. Therefore, thesource power process window in the invention is nearly an order ofmagnitude wider than that of a conventional inductively coupled reactor,a significant advantage.

Dual Advantages of the Invention

The constriction of the torroidal plasma current path in the vicinity ofthe wafer or workpiece produces two independent advantages without anysignificant tradeoffs of other performance criteria: (1) the plasmadensity over the wafer is increased without requiring any increase inplasma source power, and (2) the etch selectivity to photoresist orother materials is increased, as explained above. It is believed that inprior plasma reactors it has been impractical if not impossible toincrease the plasma ion density by the same step that increases etchselectivity. Thus, the dual advantages realized with the torroidalplasma source of the present invention appear to be a revolutionarydeparture from the prior art.

Other Preferred Embodiments

FIG. 9 illustrates a modification of the embodiment of FIG. 1 in whichthe side antenna 170 is replaced by a smaller antenna 910 that fitsinside the empty space between the ceiling 110 and the hollow conduit150. Preferably, the antenna 910 is a single coil winding centered withrespect to the hollow conduit 150.

FIGS. 10 and 11 illustrate how the embodiment of FIG. 1 may be enhancedby the addition of a closed magnetically permeable core 1015 thatextends through the space between the ceiling 110 and the hollow conduit150. The core 1015 improves the inductive coupling from the antenna 170to the plasma inside the hollow conduit 150.

Impedance match may be achieved without the impedance match circuit 175by using, instead, a secondary winding 1120 around the core 1015connected across a tuning capacitor 1130. The capacitance of the tuningcapacitor 1130 is selected to resonate the secondary winding 1120 at thefrequency of the RF power source 180. For a fixed tuning capacitor 1130,dynamic impedance matching may be provided by frequency tuning and/or byforward power servoing.

FIG. 12 illustrates an embodiment of the invention in which a hollowtube enclosure 1250 extends around the bottom of the reactor andcommunicates with the interior of the chamber through a pair of openings1260, 1265 in the bottom floor of the chamber. A coil antenna 1270follows along side the torroidal path provided by the hollow tubeenclosure 1250 in the manner of the embodiment of FIG. 1. While FIG. 12shows the vacuum pump 135 coupled to the bottom of the main chamber, itmay just as well be coupled instead to the underlying conduit 1250.

FIG. 13 illustrates a variation of the embodiment of FIGS. 10 and 11, inwhich the antenna 170 is replaced by an inductive winding 1320surrounding an upper section of the core 1015. Conveniently, the winding1320 surrounds a section of the core 1015 that is above the conduit 150(rather than below it). However, the winding 1320 can surround anysection of the core 1015.

FIG. 14 illustrates an extension of the concept of FIG. 13 in which asecond hollow tube enclosure 1450 runs parallel to the first hollowconduit 150 and provides a parallel torroidal path for a secondtorroidal plasma current. The tube enclosure 1450 communicates with thechamber interior at each of its ends through respective openings in theceiling 110. A magnetic core 1470 extends under both tube enclosures150, 1450 and through the coil antenna 170.

FIG. 15 illustrates an extension of the concept of FIG. 14 in which anarray of parallel hollow tube enclosures 150 a, 150 b, 150 c, 150 dprovide plural torroidal plasma current paths through the reactorchamber. In the embodiment of FIG. 15, the plasma ion density iscontrolled independently in each individual hollow conduit 150 a-d by anindividual coil antenna 170 a-d, respectively, driven by an independentRF power source 180 a-d, respectively. Individual cylindrical open cores1520 a-1520 d may be separately inserted within the respective coilantennas 170 a-d. In this embodiment, the relative center-to-edge iondensity distribution may be adjusted by separately adjusting the powerlevels of the individual RF power sources 180 a-d.

FIG. 16 illustrates a modification of the embodiment of FIG. 15 in whichthe array of tube enclosures 150 a-d extend through the side wall of thereactor rather than through the ceiling 110. Another modificationillustrated in FIG. 16 is the use of a single common magnetic core 1470adjacent all of the tube enclosures 150 a-d and having the antenna 170wrapped around it so that a single RF source excites the plasma in allof the tube enclosures 150 a-d.

FIG. 17A illustrates a pair of orthogonal tube enclosures 150-1 and150-2 extending through respective ports in the ceiling 110 and excitedby respective coil antennas 170-1 and 170-2. Individual cores 1015-1 and1015-2 are within the respective coil antennas 170-1 and 170-2. Thisembodiment creates two mutually orthogonal torroidal plasma currentpaths over the wafer 120 for enhanced uniformity. The two orthogonaltorroidal or closed paths are separate and independently powered asillustrated, but intersect in the process region overlying the wafer,and otherwise do not interact. In order to assure separate control ofthe plasma source power applied to each one of the orthogonal paths, thefrequency of the respective RF generators 180 a, 180 b of FIG. 17 aredifferent, so that the operation of the impedance match circuits 175 a,175 b is decoupled. For example, the RF generator 180 a may produce anRF signal at 11 MHz while the RF generator 180 b may produce an RFsignal at 12 MHz. Alternatively, independent operation may be achievedby offsetting the phases of the two RF generators 180 a, 180 b.

FIG. 17B illustrates how radial vanes 181 may be employed to guide thetorroidal plasma currents of each of the two conduits 150-1, 150-2through the processing region overlying the wafer support. The radialvanes 181 extend between the openings of each conduit near the sides ofthe chamber up to the edge of the wafer support. The radial vanes 181prevent diversion of plasma from one torroidal path to the othertorroidal path, so that the two plasma currents only intersect withinthe processing region overlying the wafer support.

Embodiments Suitable for Large Diameter Wafers

In addition to the recent industry trends toward smaller device sizesand higher device densities, another trend is toward greater waferdiameters. For example, 12-inch diameter wafers are currently enteringproduction, and perhaps larger diameter wafers will be in the future.The advantage is greater throughput because of the large number ofintegrated circuit die per wafer. The disadvantage is that in plasmaprocessing it is more difficult to maintain a uniform plasma across alarge diameter wafer. The following embodiments of the, presentinvention are particularly adapted for providing a uniform plasma iondensity distribution across the entire surface of a large diameterwafer, such as a 12-inch diameter wafer.

FIGS. 18 and 19 illustrate a hollow tube enclosure 1810 which is a wideflattened rectangular version 1850 of the hollow conduit 150 of FIG. 1that includes an insulating gap 1852. This version produces a wide“belt” of plasma that is better suited for uniformly covering a largediameter wafer such as a 12-inch diameter wafer or workpiece. The widthW of the tube enclosure and of the pair of openings 1860, 1862 in theceiling 110 preferably exceeds the wafer by about 5% or more. Forexample, if the wafer diameter is 10 inches, then the width W of therectangular tube enclosure 1850 and of the openings 1860, 1862 is about11 inches. FIG. 20 illustrates a modified version 1850′ of therectangular tube enclosure 1850 of FIGS. 18 and 19 in which a portion1864 of the exterior tube enclosure 1850 is constricted. However, theunconstricted version of FIGS. 18 and 19 is preferred.

FIG. 20 further illustrates the optional use of focusing magnets 1870 atthe transitions between the constricted and unconstricted portions ofthe enclosure 1850. The focusing magnets 1870 promote a better movementof the plasma between the constricted and unconstricted portions of theenclosure 1850, and specifically promote a more uniform spreading out ofthe plasma as it moves across the transition between the constrictedportion 1864 and the unconstricted portion of the tube enclosure 1850.

FIG. 21 illustrates how plural cylindrical magnetic cores 2110 may beinserted through the exterior region 2120 circumscribed by the tubeenclosure 1850. The cylindrical cores 2110 are generally parallel to theaxis of symmetry of the tube enclosure 1850. FIG. 22 illustrates amodification of the embodiment of FIG. 21 in which the cores 2110 extendcompletely through the exterior region 2120 surrounded by the tubeenclosure 1850 are replaced by pairs of shortened cores 2210, 2220 inrespective halves of the exterior region 2120. The side coils 165, 186are replaced by a pair of coil windings 2230, 2240 surrounding therespective core pairs 2210, 2220. In this embodiment, the displacement Dbetween the core pairs 2210, 2220 may be changed to adjust the iondensity near the wafer center relative to the ion density at the wafercircumference. A wider displacement D reduces the inductive couplingnear the wafer center and therefore reduces the plasma ion density atthe wafer center. Thus, an additional control element is provided forprecisely adjusting ion density spatial distribution across the wafersurface. FIG. 23 illustrates a variation of the embodiment of FIG. 22 inwhich the separate windings 2230, 2240 are replaced by a single centerwinding 2310 centered with respect to the core pairs 2210, 2220.

FIGS. 24 and 25 illustrate an embodiment providing even greateruniformity of plasma ion density distribution across the wafer surface.In the embodiment of FIGS. 24 and 25, two torroidal plasma current pathsare established that are transverse to one another and preferably aremutually orthogonal. This is accomplished by providing a second widerectangular hollow enclosure 2420 extending transversely and preferablyorthogonally relative to the first tube enclosure 1850. The second tubeenclosure 2420 communicates with the chamber interior through a pair ofopenings 2430, 2440 through the ceiling 110 and includes an insulatinggap 2452. A pair of side coil windings 2450, 2460 along the sides of thesecond tube enclosure 2420 maintain a plasma therein and are driven by asecond RF power supply 2470 through an impedance match circuit 2480. Asindicated in FIG. 24, the two orthogonal plasma currents coincide overthe wafer surface and provide more uniform coverage of plasma over thewafer surface. This embodiment is expected to find particularlyadvantageous use for processing large wafers of diameters of 10 inchesand greater.

As in the embodiment of FIG. 17, the embodiment of FIG. 24 creates twomutually orthogonal torroidal plasma current paths over the wafer 120for enhanced uniformity. The two orthogonal torroidal or closed pathsare separate and independently powered as illustrated, but intersect inthe process region overlying the wafer, and otherwise do not interact orotherwise divert or diffuse one another. In order to assure separatecontrol of the plasma source power applied to each one of the orthogonalpaths, the frequency of the respective RF generators 180, 2470 of FIG.24 are different, so that the operation of the impedance match circuits175, 2480 is decoupled. For example, the RF generator 180 may produce anRF signal at 11 MHz while the RF generator 2470 may produce an RF signalat 12 MHz. Alternatively, independent operation may be achieved byoffsetting the phases of the two RF generators 180, 2470.

FIG. 26 illustrates a variation of the embodiment of FIG. 18 in which amodified rectangular enclosure 2650 that includes an insulating gap 2658communicates with the chamber interior through the chamber side wall 105rather than through the ceiling 110. For this purpose, the rectangularenclosure 2650 has a horizontal top section 2652, a pair of downwardlyextending legs 2654 at respective ends of the top section 2652 and apair of horizontal inwardly extending legs 2656 each extending from thebottom end of a respective one of the downwardly extending legs 2654 toa respective opening 2670, 2680 in the side wall 105.

FIG. 27 illustrates how a second rectangular tube enclosure 2710including an insulating gap 2752 may be added to the embodiment of FIG.26, the second tube enclosure 2710 being identical to the rectangulartube enclosure 2650 of FIG. 26 except that the rectangular tubeenclosures 2650, 2710 are mutually orthogonal (or at least transverse toone another). The second rectangular tube enclosure communicates withthe chamber interior through respective openings through the side wall105, including the opening 2720. Like the embodiment of FIG. 25, thetube enclosures 2650 and 2710 produce mutually orthogonal torroidalplasma currents that coincide over the wafer surface to provide superioruniformity over a broader wafer diameter. Plasma source power is appliedto the interior of the tube enclosures through the respective pairs ofside coil windings 165, 185 and 2450, 2460.

FIG. 28A illustrates how the side coils 165, 185, 2450, 2460 may bereplaced (or supplemented) by a pair of mutually orthogonal interiorcoils 2820, 2840 lying within the external region 2860 surrounded by thetwo rectangular tube enclosures 2650, 2710. Each one of the coils 2820,2840 produces the torroidal plasma current in a corresponding one of therectangular tube enclosures 2650, 2710. The coils 2820, 2840 may bedriven completely independently at different frequencies or at the samefrequency with the same or a different phase. Or, they may be driven atthe same frequency but with a phase difference (i.e., 90 degrees) thatcauses the combined torroidal plasma current to rotate at the sourcepower frequency. In this case the coils 2820, 2840 are driven with thesin and cosine components, respectively, of a common signal generator2880, as indicated in FIG. 28A. The advantage is that the plasma currentpath rotates azimuthally across the wafer surface at a rotationalfrequency that exceeds the plasma ion frequency so that non-uniformitiesare better suppressed than in prior art methods such as MERIE reactorsin which the rotation is at a much lower frequency.

Referring now to FIG. 28B, radial adjustment of plasma ion density maybe generally provided by provision of a pair of magnetic cylindricalcores 2892, 2894 that may be axially moved toward or away from oneanother within the coil 2820 and a pair of magnetic cylindrical cores2896, 2898 that may be axially moved toward or away from one anotherwithin the coil 2840. As each pair of cores is moved toward one another,inductive coupling near the center of each of the orthogonal plasmacurrents is enhanced relative to the edge of the current, so that plasmadensity at the wafer center is generally enhanced. Thus, thecenter-to-edge plasma ion density may be controlled by moving the cores2892, 2894, 2896, 2898.

FIG. 29 illustrates an alternative embodiment of the invention in whichthe two tube enclosures 2650, 2710 are merged together into a singleenclosure 2910 that extends 360 degrees around the center axis of thereactor that constitutes a single plenum. In the embodiment of FIG. 29,the plenum 2910 has a half-dome lower wall 2920 and a half-dome upperwall 2930 generally congruent with the lower wall 2920. The plenum 2910is therefore the space between the upper and lower half-dome walls 2920,2930. An insulating gap 2921 may extend around the upper dome wall 2920and/or an insulating gap 2931 may extend around the lower dome wall2930. The plenum 2910 communicates with the chamber interior through anannular opening 2925 in the ceiling 110 that extends 360 degrees aroundthe axis of symmetry of the chamber.

The plenum 2910 completely encloses a region 2950 above the ceiling 110.In the embodiment of FIG. 29, plasma source power is coupled into theinterior of the plenum 2910 by a pair of mutually orthogonal coils 2960,2965. Access to the coils 2960, 2965 is provided through a verticalconduit 2980 passing through the center of the plenum 2910. Preferably,the coils 2960, 2965 are driven in quadrature as in the embodiment ofFIG. 28 to achieve an azimuthally circulating torroidal plasma current(i.e., a plasma current circulating within the plane of the wafer. Therotation frequency is the frequency of the applied RF power.Alternatively, the coils 2960, 2965 may be driven separately atdifferent frequencies. FIG. 30 is a top sectional view of the embodimentof FIG. 29. FIGS. 31A and 31B are front and side sectional views,respectively, corresponding to FIG. 30.

The pair of mutually orthogonal coils 2960, 2965 may be replaced by anynumber n of separately driven coils with their winding axes disposed at360/n degrees apart. For example, FIG. 32 illustrates the case where thetwo coils 2960, 2965 are replace by three coils 3210, 3220, 3230 withwinding axes placed at 120 degree intervals and driven by threerespective RF supplies 3240, 3250, 3260 through respective impedancematch circuits 3241, 3251, 3261. In order to produce a rotatingtorroidal plasma current, the three windings 3210, 3220, 3230 are driven120 degrees out of phase from a common power source 3310 as illustratedin FIG. 33. The embodiments of FIGS. 32 and 33 are preferred over theembodiment of FIG. 29 having only two coils, since it is felt much ofthe mutual coupling between coils would be around rather than throughthe vertical conduit 2980.

FIG. 34 illustrates an embodiment in which the three coils are outsideof the enclosed region 2950, while their inductances are coupled intothe enclosed region 2950 by respective vertical magnetic cores 3410extending through the conduit 2980. Each core 3410 has one end extendingabove the conduit 2980 around which a respective one of the coils 3210,3220, 3230 is wound. The bottom of each core 3410 is inside the enclosedregion 2950 and has a horizontal leg. The horizontal legs of the threecores 3410 are oriented at 120 degree intervals to provide inductivecoupling to the interior of the plenum 2910 similar to that provided bythe three coils inside the enclosed region as in FIG. 32.

The advantage of the flattened rectangular tube enclosures of theembodiments of FIGS. 18-28 is that the broad width and relatively lowheight of the tube enclosure forces the torroidal plasma current to be awide thin belt of plasma that more readily covers the entire surface ofa large diameter wafer. The entirety of the tube enclosure need not beof the maximum width. Instead the outer section of the tube enclosurefarthest from the chamber interior may be necked down, as discussedabove with reference to the embodiment of FIG. 20. In this case, it ispreferable to provide focusing magnets 1870 at the transition cornersbetween the wide portion 1851 and the narrow section 1852 to force theplasma current exiting the narrow portion 1852 to spread entirely acrossthe entire width of the wide section 1851. If it is desired to maximizeplasma ion density at the wafer surface, then it is preferred that thecross-sectional area of the narrow portion 1852 be at least nearly asgreat as the cross-sectional area of the wide portion 1851. For example,the narrow portion 1852 may be a passageway whose height and width areabout the same while the wide portion 1851 may have a height that isless than its width.

The various embodiments described herein with air-core coils (i.e.,coils without a magnetic core) may instead employ magnetic-cores, whichcan be the open-magnetic-path type (“rod” type cores) or theclosed-magnetic-core type illustrated in the accompanying drawings.Furthermore, the various embodiments described herein having two or moretoroidal paths driven with different RF frequencies may instead bedriven with same frequenct, and with the same or different phases.

FIG. 35 is a version of the embodiment of FIG. 17 in which the mutuallytransverse hollow conduits are narrowed as in the embodiment of FIG. 20.

FIG. 36 is a version of the embodiment of FIG. 24 but employing a pairof magnetic cores 3610, 3620 with respective windings 3630, 3640therearound for connection to respective RF power sources.

FIG. 37 is an embodiment corresponding to that of FIG. 35 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber. Having a number of symmetrically disposed conduitsand reentrant ports greater than two (as in the embodiment of FIG. 37)is believed to be particularly advantageous for processing wafers ofdiameter of 300 mm and greater.

FIG. 38 is an embodiment corresponding to that of FIG. 38 but havingthree instead of two reentrant conduits with a total of six reentrantports to the chamber.

FIG. 39 is an embodiment corresponding to that of FIG. 35 in which theexternal conduits join together in a common plenum 3910.

FIG. 40 is an embodiment corresponding to that of FIG. 36 in which theexternal conduits join together in a common plenum 4010.

FIG. 41 is an embodiment corresponding to that of FIG. 37 in which theexternal conduits join together in a common plenum 4110.

FIG. 42 is an embodiment corresponding to that of FIG. 38 in which theexternal conduits join together in a common plenum 4210.

FIG. 43 is an embodiment corresponding to that of FIG. 17 in which theexternal conduits join together in a common plenum 4310.

Advantageous Features of the Invention

The reactor of the invention affords numerous opportunities forincreasing etch selectivity without sacrificing other performancefeatures such as etch rate. For example, constricting the torroidalplasma current in the vicinity of the wafer not only improves etchselectivity but at the same time increases the etch rate by increasingthe plasma ion density. It is believed no prior reactor has increasedetch selectivity by the same mechanism that increases etch rate orplasma ion density over the workpiece.

Improving etch selectivity by constricting the torroidal plasma currentin the vicinity of the wafer or workpiece can be achieved in theinvention in any one of several ways. One way is to reduce thepedestal-to-ceiling or wafer-to-ceiling height. Another is to introducea gas, distribution plate or showerhead over the wafer that constrictsthe path of the torroidal plasma ion current. Another way is to increasethe RF bias power applied to the wafer or workpiece. Any one or anycombination of the foregoing ways of improving etch selectivity may bechosen by the skilled worker in carrying out the invention.

Etch selectivity may be further improved in the invention by injectingthe reactive process gases locally (i.e., near the wafer or workpiece)while injecting an inert diluent gas (e.g., Argon) remotely (i.e., intothe conduit or plenum). This is preferably accomplished by providing agas distribution plate or showerhead directly over and facing theworkpiece support and introducing the reactive process gas exclusively(or at least predominantly) through the showerhead. Concurrently, thediluent gas is injected into the conduit well away from the processregion overlying the wafer or workpiece. The torroidal plasma currentthus becomes not only a source of plasma ions for reactive ion etchingof materials on the wafer but, in addition, becomes an agent forsweeping away the reactive process gas species and theirplasma-dissociated progeny before the plasma-induced dissociationprocess is carried out to the point of creating an undesirable amount offree fluorine. This reduction in the residence time of the reactiveprocess gas species enhances the etch selectivity relative tophotoresist and other materials, a significant advantage.

The invention provides great flexibility in the application of RF plasmasource power to the torroidal plasma current. As discussed above, poweris typically inductively coupled to the torroidal plasma current by anantenna. In many embodiments, the antenna predominantly is coupled tothe external conduit or plenum by being close or next to it. Forexample, a coil antenna may extend alongside the conduit or plenum.However, in other embodiments the antenna is confined to the regionenclosed between the conduit or plenum and the main reactor enclosure(e.g., the ceiling). In the latter case, the antenna may be consideredto be “under” the conduit rather than alongside of it. Even greaterflexibility is afford by embodiments having a magnetic core (or cores)extending through the enclosed region (between the conduit and the mainchamber enclosure) and having an extension beyond the enclosed region,the antenna being wound around the core's extension. In this embodimentthe antenna is inductively coupled via the magnetic core and thereforeneed not be adjacent the torroidal plasma current in the conduit. In onesuch embodiment, a closed magnetic core is employed and the antenna iswrapped around the section of the core that is furthest away from thetorroidal plasma current or the conduit. Therefore, in effect, theantenna may be located almost anywhere, such as a location entirelyremote from the plasma chamber, by remotely coupling it to the torroidalplasma current via a magnetic core.

Finally, the invention provides uniform coverage of the plasma over thesurface of a very large diameter wafer or workpiece. This isaccomplished in one embodiment by shaping the torroidal plasma currentas a broad plasma belt having a width preferably exceeding that of thewafer. In another embodiment, uniformity of plasma ion density acrossthe wafer surface is achieved by providing two or more mutuallytransverse or orthogonal torroidal plasma currents that intersect in theprocess region over the wafer. The torroidal plasma currents flow indirections mutually offset from one another by 360/n. Each of thetorroidal plasma currents may be shaped as a broad belt of plasma tocover a very large diameter wafer. Each one of the torroidal plasmacurrents may be powered by a separate coil antenna aligned along thedirection of the one torroidal plasma current. In one preferredembodiment, uniformity is enhanced by applying RF signals of differentphases to the respective coil antennas so as to achieve a rotatingtorroidal plasma current in the process region overlying the wafer. Inthis preferred embodiment, the optimum structure is one in which thetorroidal plasma current flows in a circularly continuous plenumcommunicating with the main chamber portion through a circularlycontinuous annular opening in the ceiling or side wall. This latterfeature allows the entire torroidal plasma current to rotate azimuthallyin a continuous manner.

While the invention has been described in detail by specific referenceto preferred embodiments, it is understood that variations andmodifications thereof may be made without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A method of processing a workpiece in a plasmareactor, comprising: establishing a torroidal path for a plasma currentto flow that passes near and transverse to the surface of saidworkpiece; maintaining a plasma current in said torroidal path byapplying RF power to a portion of said torroidal path away from saidsurface of said workpiece; increasing the ion density of the plasmacurrent in the vicinity of said workpiece by constricting the area of aportion of said torroidal path overlying said workpiece.
 2. The methodof claim 1 wherein the step of constricting comprises increasing thelevel of an RF bias signal applied to said workpiece.
 3. The method ofclaim 1 wherein the step of constricting comprises introducing amechanical constriction in said torroidal path over said workpiece. 4.The method of claim 3 wherein said mechanical constriction is a gasdistribution apparatus facing said workpiece.
 5. The method of claim 4further comprising: introducing a reactive gas into said reactorexclusively through said gas distribution apparatus; introducing adiluent gas into a portion of said torroidal path distant from saidworkpiece.
 6. A method of processing a workpiece in a plasma reactor,comprising: establishing a torroidal path for a plasma current to flowthat passes near and transverse to the surface of said workpiece;maintaining a plasma current in said torroidal path by applying RF powerto a portion of said torroidal path away from said surface of saidworkpiece; introducing a reactive process gas into said plasma current;reducing the residency time of said reactive gas in said plasma.
 7. Themethod of claim 6 wherein the step of reducing the residency timecomprises: increasing the power of a bias RF signal applied to saidworkpiece so as to increase the thickness of a plasma sheath around saidworkpiece.
 8. The method of claim 6 wherein the step of reducing theresidency time comprises: introducing a diluent gas into a portion ofsaid plasma current distant from said workpiece while introducing saidreactive gas into a process region overlying said workpiece, whereby thediluent gas in said plasma current sweeps away reactive species from theprocess region.